1. Field of the Invention
Embodiments of the invention relate to methods and apparatus generally directed to the field of image processing and optical aberration measurement and characterization. More particularly, embodiments of the invention relate to methods and apparatus directed to acquiring and processing image data generated by an optical aberration measurement apparatus that utilizes a lenslet array for providing wavefront aberration data.
2. Background Art
The accurate measurement of higher-order wavefront aberrations is crucial in the fields of astronomy and ophthalmology. In astronomical applications, the intervening atmosphere between the object and the measuring device distorts the light that forms the image of the object. The ability to measure the distortion (aberrations) allows for compensation correction of the imaging system to produce a well defined image.
In an ocular system, light from an object gets distorted by the cornea and crystalline lens on its path to the retina where the image is formed. The ability to measure the ocular aberrations makes it possible to correct the image forming system through surgery and/or corrective lens fabrication. Ultra-accurate aberration measurement and correction has provided super-normal vision to many individuals.
Although various types of aberration measurement apparatus (hereinafter, “aberrometer”) are known, Hartmann-Shack type aberrometers are widely used in commercial ophthalmic applications. In a Hartmann-Shack type device, a beam of light from a laser diode or other light source is directed toward the pupil and is incident on the retina. The aberrated light exiting the pupil is applied to a Hartmann-Shack sensor, which includes an array of lenslets that form an array of aerial image spots and focus the spots onto a detector. The location of each detected image spot relative to its position in the absence of wavefront aberrations provides data that is used to reconstruct the wavefront and thus detect the aberrations.
A seminal reference in the field of ophthalmic wavefront detection is Liang et al., Objective measurement of wave aberrations of the human eye with the use of a Hartmann-Shack wave-front sensor, Journal of the Optical Society of America, Vol. 11, No. 7, pp. 1-9 (July 1994), the disclosure of which is hereby incorporated by reference in its entirety. Improvements to the technique of Liang et al., id., are taught in Liang and Williams, Aberrations and retinal image quality of the normal human eye, Journal of the Optical Society of America, Vol. 4, No. 11, pp. 2873-2883 (November 1997), and in Williams et al. U.S. Pat. No. 5,777,719, the disclosures of which are hereby incorporated by reference in their entireties.
The ability to accurately measure higher-order aberrations and use the measurement information in corrective applications critically depends, in turn, on the ability to precisely determine the location of the detected image spots as well as their displacement. However, many known factors operate to frustrate improved and consistent measurement precision and accuracy. For example, scattered light can form ghost images and/or create background noise on the detector that interferes with actual image spot detection. Image processing techniques that employ high band-pass filtering or certain linear filters may create significant edge distortion and/or may radically alter the size and shape of a feature of the image. Other problems may be that one or more image spots that should be formed in the Hartmann-Shack detector cannot be seen because the aberrations may be too large (a scar, surface flaw, or keratoconus, for example, may displace or deform the spot so much that the spot's origin (centroid) cannot be determined, or the spot leaves the field of view of the detector altogether); or, they are occluded by the subject's eyelid, ocular opacities, or a host of other reasons. Typical aberrometers are also plagued by artifacts caused by internal reflections from inside the instrument, particularly from the faces of the internal reflective mirrors. Furthermore, the subject's myopia or hyperopia (lower-order defocus aberrations), if not properly corrected, may overwhelm the dynamic range of the Hartmann-Shack sensor and prevent the detection of some or all image spots.
Even if the image spots are detected, centroid calculation algorithms may provide inaccurate results. The traditional centroid calculation algorithm is derived from basic calculus. Center coordinates cx, cy of a region R of an image spot are calculated by summation of weighted values of the incident light intensity I(xi, yi) at points (xi, yi) in R. However, there are several pitfalls in the traditional application of the standard centroid calculation algorithm. For example, a) if an integration region is not centered around the central point, then the computed center will tend toward the direction of miscentering. This problem is aggravated by increasing background scatter noise; b) if more than one spot, or the reflection of a spot, is included in the calculation, then the computation will yield a center somewhere between the two spots, which will be erroneous; c) if the integration region is insufficiently large, then the calculation will have lowered precision; d) due to problematic multiple peaks in the image, traditional approaches assume that the spot center will be found in a given region. These algorithms fail for pathological eyes with severe aberrations.
Indeed, the inability to precisely and accurately detect the image spots frustrates the computation of the wave aberration and subsequent applications that rely upon those computations.